Celgene Corp. CELG, -2.40% shares added to Monday's losses in the extended session after the biotech said its president and chief operating officer, Scott Smith, was leaving the company immediately. Celgene shares slipped 0.6% after hours, following a 2.4% decline to finish at $87.07 Monday. The company said that Mark Alles, Celgene's chairman and chief executive, will assume Smith's duties.